Adsorption behavior and neurotoxic synergy of thallium and polystyrene microplastics in Caenorhabditis elegans

Microplastics (MPs) have emerged as ubiquitous environmental contaminants, while thallium (Tl), a highly toxic metalloid, is gaining attention as a novel pollutant due to its increasing release from electronic waste and mining activities. These pollutants frequently coexist in aquatic environments; however, their combined effects at environmentally relevant concentrations remain poorly understood. In this study, the adsorption behavior and joint neurotoxicity of polystyrene (PS) microplastics and Tl were systematically evaluated using Caenorhabditis elegans as a model organism. Adsorption kinetics followed both pseudo-first-order and pseudo-second-order models, yielding maximum Tl adsorption capacities of 66.682 µg/g and 67.981 µg/g, respectively. Adsorption efficiency declined with increasing salinity but was enhanced by higher pH, temperature, and humic acid (HA) concentrations. Neurotoxicity assays were conducted using Tl at 0.01 and 0.1 μg/L, along with PS at corresponding Tl-saturated adsorption concentrations (147 and 1470 μg/L). Behavioral analysis revealed that PS significantly amplified Tl-induced neurotoxicity, as evidenced by reductions in head thrashes and body bends. Co-exposure led to pronounced neurodegeneration and altered fluorescence intensity in serotonergic, GABAergic, and glutamatergic neurons in transgenic nematodes. Additionally, neurotransmitter levels were markedly decreased, and the expression of key neurofunctional genes (e.g., mod-1, tph-1, and unc-46) was significantly dysregulated. Collectively, these findings demonstrate that PS microplastics potentiate the neurotoxic effects of Tl by disrupting multiple neurotransmission pathways, underscoring the ecological risks associated with the co-occurrence of MPs and heavy metals in the environment.